Optical components having flat and spherical surfaces are relatively easy of manufacturing and measuring. However, due to their limited performance by aberrations, they are being replaced with optical components having complex surfaces, such as aspherical and freeform surfaces. Optical components with complex surfaces allow for elimination of aberrations and provide many other advantages. They are obtained through the use of machining, polishing and metrology techniques.
In more recent years, advanced manufacturing technologies for such optical components have been developed due to a demand for ever-complex optical surfaces to be machined. Metrology requirements for such optical components are therefore becoming increasingly higher.
Surface metrology for surface characterization is currently carried out through different techniques such as contact measurement, scanning probe measurement and non-contact measurement.
Contact measurement is based on the use of a small stylus tip that is dragged across the sample. Peaks and valleys on the sample surface are tracked by the tip. The up and down movement of the tip is converted into a signal that is processed into digital data useful for building a curve related to the position in the sample versus the height at that point of the sample.
A constant force has to be exerted by the tip on the sample surface in order to correctly follow its shape. Therefore, a main disadvantage of contact based measurement is that it is destructive since the measuring tip is always in contact with the sample being measured. A further disadvantage of contact measurement techniques is that, although currently tips under the micrometer scale can be manufactured, details smaller than the dimension of the tip itself cannot be measured.
Scanning probe measurement is based on the use of a probe that is operated close to the surface to be measured at a distance where the forces between the probe and the surface are present. Atomic force microscopy is used in this technique for measuring aspheric surfaces.
Non-contact measurement can be carried out through interferometry, confocal profilometry or laser autofocus techniques.
In the interferometry technique, interference fringes are assessed when a reference beam overlaps in the observation plane over a beam carrying information on the surface that is measured. Phase differences in the reflected beam are measured and converted into height information such that a surface profile can be obtained.
Through the use of interferometers, information is obtained on the topography of the sample surface that is measured. The wavefront from the sample is compared to a reference wavefront such that the overlap in a detector, usually a camera, results in an interference pattern from which the difference in height between measurement surface and a reference surface (usually a flat mirror) can be assessed. For measuring aspheric surfaces the reference wavefront has to be modified to be as similar as possible to the measurement wavefront. This is necessary in order to perform the measurement due to the nature of the interferometric signal. When the phase difference between the two wavefronts is large the fringes in the interference pattern are so compressed that the camera can not resolve them and some information is lost. The technique of modifying the wavefront reference to make it similar to the wavefront measurement is referred to as nulling. For spherical or flat samples, it is easy to get a spherical or flat reference wavefront. However, for aspheric surfaces it is much more complex as an aspherical reference wavefront is required.
Confocal profilometry is performed by confocal microscopes in which the sample is illuminated through a very small pinhole and observed with a photodetector placed after a second small pinhole. In this way, only the light exactly coming from the focus plane will reach the photodetector. By rejecting out of focus light, the image then comes from a thin section of the object (small depth of field) such that by scanning many thin sections through the object a very clean three-dimensional image thereof can be built up.
Confocal depth sectioning can also be attained using structured patterns of light projected on a sample and depth-of-focus algorithms.
WO9000754 shows an approach to confocal microscopy. A confocal microscope comprises a light source for supplying a light beam to a lens that focuses the light onto an object to be examined so as to illuminate a point observational field on or within the object. Reflected light from the illuminated point field is collected by condenser and transmitted to detector. Scanning means causes illuminated point field to move in scanning pattern relative to object. The outgoing light passing from light source to condenser and the returning light are transmitted via optical fibers and a light separator to divert the return light to detector.
Laser autofocus technique can be considered as the optical equivalent to the contact measurement technique. In this case, however, the stylus tip is replaced by an optical tip, that is, the focal point of a microscope objective lens. In one example of laser autofocus technique, an off-axis laser beam is output on the sample under analysis through microscope objective lens. If the sample is at the focal point the laser beam is reflected reaching a detector center. If the sample is out of focus the laser beam will reach another region of the detector. As this is detected, the objective lens is moved so that the laser beam impinges again on the detector center. For performing a measurement, this technique is based on moving the optical tip that represents the laser beam across the sample and refocusing the sample at each position so that the distance between the measuring system and the sample is kept always constant (autofocus). Information on the surface topography is thus obtained from the movement performed by the system along the sample in order to keep it always in focus.
The above metrology techniques allow the shape and the texture of complex surfaces to be accurately assessed. Data acquired are compared with design data in order to obtain correction data that is then delivered to polishing systems. Said acquired data correspond to shape specifications in the range of from tens to hundreds of nanometers.
The shape of an aspheric surface is given by the following expression:
      Z    ⁡          (      X      )        =                              X          2                /        R                    1        +                              1            -                                          (                                  K                  +                  1                                )                            ⁢                                                X                  2                                                  R                  2                                                                          +                  A        4            ⁢              X        4              +                  A        6            ⁢              X        6              +                  A        8            ⁢              X        8              +                  A        10            ⁢              X        10            wherein Z is the topographic coordinate (height) and X is the lateral coordinate. R and K are respectively the radius of curvature and the conic constant and the parameters A4, A6, A8, A10, etc. are asphericity coefficients. The metrological requirements for Z(x) are of the order of λ/25 (≈10 nm) for measurement repeatability and λ/10 (≈50 nm) for maximum shape error (accuracy).
No effective solution has been however provided by the prior art so far to the problem of measuring these surfaces through non-contact techniques. This is mainly due to the strong nonlinearity of the aspheric terms X2n, which results in that measuring becomes much more critical at points X that are farther from the apex of the surfaces (X=0). The points where the reliability of the measure must be high are the ones in steeper slope areas of the optical surface.
The combination of having a polished surface with a steep slope and the required accuracy at the nanoscale scale causes the metrology of these surfaces with non-contact optical techniques to be a scientific and technical challenge of the highest difficulty.
EP1555561 discloses a non-contact surface configuration measuring method for accurate measurement of a surface at a steep angle to a laser probe. Specific areas including parts inclined ±30° or more from an optimum measurement state relative to the laser probe are measured after a workpiece is rotated such that the surface within the specific areas is less than ±30°. Therefore, accurately measured data on the specific areas can be obtained even in a different coordinate system from that of a general area.
A relevant problem derived from any of the above techniques is the low speed of operation if high accuracy is desired. A further disadvantage is that these methods are non-continuous.